Fifteen people accepted avowed white supremacist Payton Gendron’s invitation to a private chatroom — chillingly named “Happening: This is not a drill” — where he allegedly spent months meticulously planning his attack in Buffalo that killed 10 people.

The 18-year-old invited people on the Discord app 30 minutes before the massacre, CNN reported.

The chatroom included months of posts written by an author who identified himself as Gendron, describing his twisted views and outlining how he’d scouted the Tops Friendly Markets in a crazed plan to kill as many black people as possible.

A person with knowledge of Discord’s internal investigation told the network that the invitees — and others with whom they may have shared access — would have been able to view his posts before the massacre.

The Washington Post reported that 15 people had accepted the invitation, citing a person with knowledge of the messaging platform’s probe.

A copy of the invite reviewed by the paper said the accepted users also could view a video stream of the shooting through a link to an account on Twitch, a video streaming service where Gendron allegedly broadcast video of his attack.

It was unclear whether Discord can determine exactly what the 15 users did after they accepted the invitation, the source told the paper